To those appreciative of the art of manufactured PM articles, the achievement of high density is of significant importance. High density generally significantly improves the strength and durability characteristics of the manufactured article. The amount of residual porosity in relation to powder metal sintered articles of low alloy steel type compositions has a profound influence on the loading conditions that the article can withstand in its operation. At high levels of residual porosity (i.e. low density) manufactured articles are brittle and of low fatigue strength. Such low density articles can generally only be used in applications where service loading is relatively light. The available market for low density PM compacts is therefore restricted. At lower levels of residual porosity (i.e. high density), the manufactured articles become ductile and of significantly greater fatigue strength. The manufacture of low alloy PM articles at relatively high density is therefore attractive because increased market share can be achieved due to improved properties of the article.
Several prior art methods and procedures such as hot forging or double pressing and double sintering for example have been developed with the objective of increasing density for the reasons referred to above. However many of these processes have drawbacks which hinder their use for the economic production of articles in high volumes. Such drawbacks may include the requirement to use high temperatures during forming, which leads to high die wear costs, and associated dimensional accuracy problems. High cost raw materials may be used, such as fine powders. For example the metal injection molding process (MIM) uses iron of about 10 microns in size which can be used to manufacture high density articles; however the economics of the process are adversely affected because of the high cost of the raw material. Processes such as hot isostatic pressing (HIP) or pressure assisted sintering (PAS) are examples where high temperatures and high gas pressures may be used during sintering. However such equipment has throughput limitations and dimensional precision is difficult to control.
For a process to be of commercial value and offer a significant improvement in durability of the sintered powdered part the method of producing high density sintered powder metal parts should meet the following criteria:
use low cost raw materials PA1 be suited to high volume production rates PA1 produce articles of high precision PA1 have acceptable tool life characteristics PA1 produce articles with a density in the range of 94% to 98% theoretical full density of wrought iron (equivalent to a range of 7.4 to 7.7 g/cc for low alloy compositions).
The use of a prealloyed powder is discussed by Yoshiaki et al in the SAE Technical Paper Series, given at the International Congress and Exposition in Detroit, Mich. on Feb. 27-Mar. 3, 1989, which is entitled "Improvement Of The Rolling Contact Fatigue Strength of Sintered Steel for Transmission Component". However, Yoshiaki does not teach the use of prealloyed molybdenum powder metal or ferro alloys or substantially pure blends or additional selective densification to produce powder metal parts having high density and ductility.
It is an object of this invention to provide an improved method to produce powder metal parts having high density and ductility.
It is an aspect of this invention to provide a method of forming sintered powder metal articles to a high density by forming the sintered powder metal in a closed die cavity having a clearance for movement of said sintered powder metal to final shape with increased density after compression, wherein the formed sintered powder metal part has a compressed length of approximately 3 to 30% less than the original length.
It is another aspect of this invention to produce a method of forming sintered powder metal article by blending carbon; at least one ferro alloy powder selected from the group of ferro chromium, ferro manganese, ferro molybdenum, and a lubricant, with iron powder to form a blended mixture; pressing the blended mixture to form the article; sintering the article at a temperature greater than 1250.degree. C.; forming the sintered article in a closed die cavity having a clearance so as to produce a formed sintered powder metal part having a compressed length which is approximately 3 to 19% less than the original length when subjected to a pressure between 40 and 90 tons per square inch so as to increase the density of the formed sintered article; annealing the formed sintered article at a temperature greater than 800.degree. C. in a reducing or carburizing atmosphere or vacuum.
It is a further aspect of this invention to provide a method of making a high density sintered powder metal article, comprising the steps of blending iron powder with ferro alloys, graphite and lubricant to provide a selected chemical composition for the finished article having at least one of the following: 0 to 0.5% carbon, 0 to 1.5% manganese, 0 to 1.5% molybdenum and 0 to 1.5% chromium and the remainder iron powder with unavoidable impurities; compacting the metal powder mixture in a rigid die to a density of approximately 90% of theoretical full density; sintering the compacted article at a temperature greater than 1250.degree. C. in a reducing atmosphere or vacuum; forming the sintered article in rigid tools at pressure in the range of 40 to 90 tons per square inch to a density in excess of 94% of theoretical full density by axial compression allowing radial expansion to decrease the axial length of the sintered article by approximately 3 to 30% of the original axial length; annealing the high density article at a temperature greater than 800.degree. C. in a reducing or carburizing atmosphere or vacuum, where the total alloy composition is between 0 to 4.0% by weight to the total weight of sintered powder metal article.
It is another aspect of this invention to provide a method of forming sintered powder metal articles by blending carbon and lubricant with a prealloyed molybdenum powder, pressing said blended mixture to form said article, sintering said article at a temperature of at least 1100.degree. C., forming the sintered powder metal article in a closed die cavity having a clearance for movement of said sintered powder metal to final shape with increased density, after compression wherein the formed sintered powder metal article has a compressed length which is 3 to 30% less than the original length.
A further aspect of this invention relates to a method of forming sintered powder metal articles to a high density by forming the sintered powder metal in a die cavity having a clearance for movement of said sintered powder metal to final shape with increased density after compaction wherein the formed sintered powder metal article has a compressed length which is approximately 3 to 30% less than the original length.
Yet another aspect of this invention relates to a formed sintered powder metal article having up to 0.5% by weight Carbon, up to 1.5% by weight Mn with the remainder being iron and unavoidable impurities and having approximately 23% elongation and density greater than 7.4 g/cc.
Another aspect of this invention relates to a method of forming sintered powder metal articles to high density by selecting a target critical diameter to achieve through hardening upon quenching of the formed sintered part, then selecting a powder composition to achieve the selected target critical diameter and density between 7.4 and 7.7 g/cc.